1. Field of the Invention
The present invention relates generally to a vacuum laser irrdiating apparatus disposed between a laser generator and a vacuum vessel in which an object to be tested or processed (cutting, boring, etc.) is placed so as to be irradiated with a laser beam, and more specifically to a vacuum laser irradiating apparatus provided with an outer window plate.
2. Description of the Prior Art
In a certain laser system, an object to be heated or processed is placed within a vacuum vessel and irradiated from outside with a laser beam generated from a laser generator via a vacuum laser irradiating apparatus having a laser transmissible window plate. In the laser system of this type, an excimer laser source such as ArF or XeF is generally utilized. The excimer laser is generated when excited molecules (so-called excimer) produced by atoms under excited conditions and the normal conditions are returned to the dissociation state, and provided with such features as a short wavelength, a high output power, and a high efficiency, etc.
Since the excimer laser is short in wavelength and high in energy, it is impossible to effectively transmit a laser beam through a window plate formed of the ordinary light transmissible material. That is, a window plate formed of a pure monocrystalline material such as calcium fluoride (CaF) or magnesium fluoride (MgF) has been conventionally used to transmit the excimer laser therethrough.
When the above-mentioned monocrystalline material is used as the window plate, however, if the monocrystalline material is mixed with impurity or polycrystalline material, since the laser energy tends to be collected to these impurity or polycrystalline substances, there exists a problem in that the window plate becomes cloudy so that a laser beam cannot be well transmitted. Therefore, it is indispensable to use a pure monocrystalline material including as small amount of impurity or polycrystal substances as possible, so that the cost of the excimer laser transmissible window plate is very high. To reduce the cost thereof, therefore, it has been necessary to reduce the thickness of the window plate as thin as possible.
In some cases, however, since the vacuum vessel is kept at an ultrahigh vacuum less than 10.sup.-8 Torr (1.33.times.10.sup.-6 Pa in international system of unit), when a thin window plate formed of expensive CaF or MgF is used, there exists a problem in that the window plate is distorted, damaged or broken at the worst due to a pressure difference between a vacuum inside the vacuum vessel or the irradiating apparatus body and the atmospheric pressure outside the irradiating apparatus. Once the window plate is distorted, since the laser beam is scattered or refracted irregularly, there exists a serious problem in that the excimer laser beam cannot be effectively irradiated upon an objected to be heated or processed.